Electroluminescent display screen and circuit therefor



E- A. SACK Dec. 6, 1966 ELECTROLUMINESCENT DISPLAY SCREEN AND CIRCUITTHEREFOR Filed Dec. 17, 1962 Voltage Across C Fig.3 RB

INVENTOR.

EDGAR A. SACK ATTORNEY United States Patent 3,290,554 ELECTRULUMINESQENTDISPLAY SCREEN AND CIRCUIT THEREFOR Edgar A. Sack, Pittsburgh, Pa,assignor, by mesne assignments, to the United States of America asrepresented by the Secretary of the Navy Filed Dec. 17, 1%2, Ser. No.245,330 1 Claim. (Cl. 315169) This invention relates to display devicesand, more particularly, to sol-id state display screens formed of anarray of electroluminescent ferroelectric elements.

In the operation of such display screens, a contro charge or signal isapplied to each element, which functions as a non-linear capacitor, andthe charge is later removed by another signal or charge. The elementsare provided with an electroluminescent material and the period duringwhich the element is charged by the control signal sets the brightnessof the element.

Ferroelectric capacitor elements have been used as the storage means forcontrolling the light from the electroluminescent material since ascreen formed of these elements provides a display having highbrightness and a minimum of flicker. The electroluminescent material orelement may be applied to or formed on the ferroelec-tric element invarious ways and emits light in accordance with an alternating or timevarying potential appearing across it, while the ferroelectric capacitorcauses this potential to be altered in accordance With the applied videocontrol potential.

This invention concerns an electroluminescent ferroelectric displayscreen formed of barium titanate crystals or square loop barium titanateceramic elements in which the control charge is supplied to the elementsin a manner to provide the elements with inherent controlled persistencecharacteristics in order that the electroluminescent material or elementmay be operated to on condition by a control signal and remain on for apre-selected period and return to the off state without the necessity ofusing an oil signal.

One object of the invention resides in the provision of a display screenformed of electroluminescent ferroelectric elements having circuitry forcontrolling the supply of control charge to the elements to provideexcitation of the electroluminescent material for a selected timeperiod.

Another object of the invention resides in the provision of a displayscreen formed of electroluminescent ferroelectric elements havingcircuitry for controlling the supply of the control charge to theelements and also for excitation of the electroluminescent material forpredetermined time periods.

Other objects, advantages and features of the invention will becomeapparent from the following detailed description of the invention whenconsidered in conjunction with the accompanying drawings, wherein:

FIG. 1 shows diagrammatically an outline of a display screen with arepresentative group of barium titanate ferroelectric elementsincorporated therein and greatly enlarged;

FIG. 2 shows the circuitry for one of the cells of FIG. 1;

FIG. 3 is a graph showing hysteresis loops for the barium titanateelements of FIG. 1;

FIG. 4 shows a wave form when the elements are of the voltage responsivetype;

FIG. 5 shows a wave form when the elements are of the current responsivetype;

FIG. 6 is a schematic showing the circuitry of FIG. 2 arranged forsupplying the charge to the representative group elements of FIG. 1.

Referring to FIG. 1, the display screen 10 shows a small group ofelements 11 which are greatly enlarged in the figure but are actually ofminiature size and a practical display screen may contain upwardly of250 elements per square inch. Regardless of the actual number ofelements used to form a screen, the functions of the elements 11 arethose of emitting light in accordance with the A.-C. potential appearingacross an element and of causing this potential to be altered inaccordance with the applied video control potential.

In FIG. 2, the circuitry for a single element 11 of the display screenis shown. In this figure, an element 11 is separated into two of itscomponent parts which are indicated at 12 and 13. The component 12represents the barium titanate ferroelectric material while thecomponent 13 represents the electroluminescent material. The circuit forthe signal charge is from the voltage source E by line 14, to line 15having resistor R therein, components 12 and 13 and return line 16 tosource E. Resistance R is in parallel with the electroluminescentmaterial 13 and represents the inherent screen resistance whichdetermines the leakage of voltage from the element 13.

Refer to FIG. 3, wherein the vertical axis represents electricaldisplacement and the horizontal axis represents voltage applied acrossthe ferroelectric component 12. Assume that the ferroelectric materialis barium titanate and point A is the on condition and point B is theoff condition, that is, point A represents the charged condition andpoint B the discharged condition. By selection of the values of pulsevoltage E and resistor R, a desired net charge may be supplied tocomponent 12 during a cycle to set the component to the point A or oncondition in order to charge this component for exciting the component13 during a discharge period.

If component 13 is voltage responsive and the charge is a voltagesufficient to excite electroluminescence, the wave form across C will beas shown in FIG. 4. The charge period 214 is made a small fraction ofthe period of electroluminescence t t and the values of E and R will beadjusted so that substantially all of the charge leaks off component 13by R during the period 1 -h.

If component 13 is current responsive, the wave form is then as shown inFIG. 5. Assume N pulses of charge to be supplied to set the material 12to on or point A of FIG. 3, then, after the material is filled, thefirst pulse thereafter finds the component 12 incapable of accepting thecharge and component 13 is excited during a frame period of succeedingpulses. The net result of this operation is that with component 12starting at point A of FIG. 3, 13 will emit N light pulses [over a timeN (t t and then goes out and stays out. Only by resetting 12 to point A,or by reversing the polarity of B, will the element again emit light.

In FIG. 6, the circuitry of FIG. 2 is shown applied schematically to thegroup of elements 11 of the screen It and accordingly a circuit is shownfor each of the 9 elements of FIG. 1. In FIG. 6', the circuits for eachhorizontal row are supplied in parallel from voltage source E which,because of the rather large resistances R, will overcome problems inscanning which may be involved if the circuits were connected in series.

The following paragraph is for the purpose of clarifying the operationof the screen '10 or elements 11.

Assume the capacitance of component 12 to he 10 micromicrofarads andthat volts must appear across 13 in order to provide the desiredbrightness. Then the charge B would be 10- farads 100 volts resulting in10- coulom'bs or 1 millimicrocoulomb of current. As sume further that arepetition rate of 1000 pulses per second is necessary for adequatelight, and if a frame time is second, .a change of 1000 p.p.s. 30 sec.

X 1 millimicroeoulomb 10 volts 10- amps or have a resistance value ofohms.

A display screen having the characteristic of controlled inherentpersistence would have particular application with conventionaltelevision or C.R.T. equipment where the frame time of the screenscanning is of fixed duration and consequently would eliminate thenecessity of equipment for turning the elements to off condition.Various modes of scanning the above described screen may be used. It maybe placed in a CRT. envelope and the electron beam, for example, used asthe current source or current diode distribution systems of the priorart may be used requiring only one half this number of diodes necessarysince only the write or on signals would need to be distributed.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It it therefore to beunderstood that within the scope of the appended claim the invention maybe practiced otherwise than as specifically described.

I claim:

In a display screen formed of a plurality of inde-pend' ently controlleddisplay elements, the improvement in which,

(a) said display elements are formed of barium titanate material havinga surface of electroluminescent material,

(b) an electrical circuit including a voltage source and a firstresistor connected in series with each of said display elements to applya charge to the barium titanate material,

(c) a second discharging resistor in said circuit and connected inparallel with the electroluminescent material,

(d) said volt-age source and first resistor being operative to supply anelectrical charge of predetermined value to the barium titanate materialand (e) said discharging resistor bypassing the barium titanate materialand having a value selected to control the rate of discharge of theelectrical energy from the barium titanate material and provide apredetermined period of excitation of the electroluminescent material.

References Cited by the Examiner UNITED STATES PATENTS 2,860,322 11/1958Stadler 340-1732 2,917,667 12/1959 Sack 315169 3,011,157 11/1961Anderson 340173.2 3,042,823 7/ 1962 Williard 3136 3,154,720 10/1964Cooperman 31569 JOHN W. HUCKERT, Primary Examiner.

A. I. JAMES, Assistant Examiner.

